from __future__ import unicode_literals

from collections import defaultdict

from django.contrib.contenttypes.models import ContentType
from django.core import checks
from django.core.exceptions import FieldDoesNotExist, ObjectDoesNotExist
from django.db import DEFAULT_DB_ALIAS, models, router, transaction
from django.db.models import DO_NOTHING, signals
from django.db.models.base import ModelBase, make_foreign_order_accessors
from django.db.models.fields.related import (
    ForeignObject, ForeignObjectRel, ReverseManyToOneDescriptor,
    lazy_related_operation,
)
from django.db.models.query_utils import PathInfo
from django.utils.encoding import python_2_unicode_compatible, smart_text
from django.utils.functional import cached_property


@python_2_unicode_compatible
class GenericForeignKey(object):
    """
    Provide a generic many-to-one relation through the ``content_type`` and
    ``object_id`` fields.

    This class also doubles as an accessor to the related object (similar to
    ForwardManyToOneDescriptor) by adding itself as a model attribute.
    """

    # Field flags
    auto_created = False
    concrete = False
    editable = False
    hidden = False

    is_relation = True
    many_to_many = False
    many_to_one = True
    one_to_many = False
    one_to_one = False
    related_model = None
    remote_field = None

    def __init__(self, ct_field='content_type', fk_field='object_id', for_concrete_model=True):
        self.ct_field = ct_field
        self.fk_field = fk_field
        self.for_concrete_model = for_concrete_model
        self.editable = False
        self.rel = None
        self.column = None

    def contribute_to_class(self, cls, name, **kwargs):
        self.name = name
        self.model = cls
        self.cache_attr = "_%s_cache" % name
        cls._meta.add_field(self, virtual=True)

        # Only run pre-initialization field assignment on non-abstract models
        if not cls._meta.abstract:
            signals.pre_init.connect(self.instance_pre_init, sender=cls)

        setattr(cls, name, self)

    def get_filter_kwargs_for_object(self, obj):
        """See corresponding method on Field"""
        return {
            self.fk_field: getattr(obj, self.fk_field),
            self.ct_field: getattr(obj, self.ct_field),
        }

    def get_forward_related_filter(self, obj):
        """See corresponding method on RelatedField"""
        return {
            self.fk_field: obj.pk,
            self.ct_field: ContentType.objects.get_for_model(obj).pk,
        }

    def __str__(self):
        model = self.model
        app = model._meta.app_label
        return '%s.%s.%s' % (app, model._meta.object_name, self.name)

    def check(self, **kwargs):
        errors = []
        errors.extend(self._check_field_name())
        errors.extend(self._check_object_id_field())
        errors.extend(self._check_content_type_field())
        return errors

    def _check_field_name(self):
        if self.name.endswith("_"):
            return [
                checks.Error(
                    'Field names must not end with an underscore.',
                    hint=None,
                    obj=self,
                    id='fields.E001',
                )
            ]
        else:
            return []

    def _check_object_id_field(self):
        try:
            self.model._meta.get_field(self.fk_field)
        except FieldDoesNotExist:
            return [
                checks.Error(
                    "The GenericForeignKey object ID references the non-existent field '%s'." % self.fk_field,
                    hint=None,
                    obj=self,
                    id='contenttypes.E001',
                )
            ]
        else:
            return []

    def _check_content_type_field(self):
        """
        Check if field named `field_name` in model `model` exists and is a
        valid content_type field (is a ForeignKey to ContentType).
        """
        try:
            field = self.model._meta.get_field(self.ct_field)
        except FieldDoesNotExist:
            return [
                checks.Error(
                    "The GenericForeignKey content type references the non-existent field '%s.%s'." % (
                        self.model._meta.object_name, self.ct_field
                    ),
                    hint=None,
                    obj=self,
                    id='contenttypes.E002',
                )
            ]
        else:
            if not isinstance(field, models.ForeignKey):
                return [
                    checks.Error(
                        "'%s.%s' is not a ForeignKey." % (
                            self.model._meta.object_name, self.ct_field
                        ),
                        hint=(
                            "GenericForeignKeys must use a ForeignKey to "
                            "'contenttypes.ContentType' as the 'content_type' field."
                        ),
                        obj=self,
                        id='contenttypes.E003',
                    )
                ]
            elif field.remote_field.model != ContentType:
                return [
                    checks.Error(
                        "'%s.%s' is not a ForeignKey to 'contenttypes.ContentType'." % (
                            self.model._meta.object_name, self.ct_field
                        ),
                        hint=(
                            "GenericForeignKeys must use a ForeignKey to "
                            "'contenttypes.ContentType' as the 'content_type' field."
                        ),
                        obj=self,
                        id='contenttypes.E004',
                    )
                ]
            else:
                return []

    def instance_pre_init(self, signal, sender, args, kwargs, **_kwargs):
        """
        Handle initializing an object with the generic FK instead of
        content_type and object_id fields.
        """
        if self.name in kwargs:
            value = kwargs.pop(self.name)
            if value is not None:
                kwargs[self.ct_field] = self.get_content_type(obj=value)
                kwargs[self.fk_field] = value._get_pk_val()
            else:
                kwargs[self.ct_field] = None
                kwargs[self.fk_field] = None

    def get_content_type(self, obj=None, id=None, using=None):
        if obj is not None:
            return ContentType.objects.db_manager(obj._state.db).get_for_model(
                obj, for_concrete_model=self.for_concrete_model)
        elif id is not None:
            return ContentType.objects.db_manager(using).get_for_id(id)
        else:
            # This should never happen. I love comments like this, don't you?
            raise Exception("Impossible arguments to GFK.get_content_type!")

    def get_prefetch_queryset(self, instances, queryset=None):
        if queryset is not None:
            raise ValueError("Custom queryset can't be used for this lookup.")

        # For efficiency, group the instances by content type and then do one
        # query per model
        fk_dict = defaultdict(set)
        # We need one instance for each group in order to get the right db:
        instance_dict = {}
        ct_attname = self.model._meta.get_field(self.ct_field).get_attname()
        for instance in instances:
            # We avoid looking for values if either ct_id or fkey value is None
            ct_id = getattr(instance, ct_attname)
            if ct_id is not None:
                fk_val = getattr(instance, self.fk_field)
                if fk_val is not None:
                    fk_dict[ct_id].add(fk_val)
                    instance_dict[ct_id] = instance

        ret_val = []
        for ct_id, fkeys in fk_dict.items():
            instance = instance_dict[ct_id]
            ct = self.get_content_type(id=ct_id, using=instance._state.db)
            ret_val.extend(ct.get_all_objects_for_this_type(pk__in=fkeys))

        # For doing the join in Python, we have to match both the FK val and the
        # content type, so we use a callable that returns a (fk, class) pair.
        def gfk_key(obj):
            ct_id = getattr(obj, ct_attname)
            if ct_id is None:
                return None
            else:
                model = self.get_content_type(id=ct_id,
                                              using=obj._state.db).model_class()
                return (model._meta.pk.get_prep_value(getattr(obj, self.fk_field)),
                        model)

        return (ret_val,
                lambda obj: (obj._get_pk_val(), obj.__class__),
                gfk_key,
                True,
                self.cache_attr)

    def is_cached(self, instance):
        return hasattr(instance, self.cache_attr)

    def __get__(self, instance, instance_type=None):
        if instance is None:
            return self

        try:
            return getattr(instance, self.cache_attr)
        except AttributeError:
            rel_obj = None

            # Make sure to use ContentType.objects.get_for_id() to ensure that
            # lookups are cached (see ticket #5570). This takes more code than
            # the naive ``getattr(instance, self.ct_field)``, but has better
            # performance when dealing with GFKs in loops and such.
            f = self.model._meta.get_field(self.ct_field)
            ct_id = getattr(instance, f.get_attname(), None)
            if ct_id is not None:
                ct = self.get_content_type(id=ct_id, using=instance._state.db)
                try:
                    rel_obj = ct.get_object_for_this_type(pk=getattr(instance, self.fk_field))
                except ObjectDoesNotExist:
                    pass
            setattr(instance, self.cache_attr, rel_obj)
            return rel_obj

    def __set__(self, instance, value):
        ct = None
        fk = None
        if value is not None:
            ct = self.get_content_type(obj=value)
            fk = value._get_pk_val()

        setattr(instance, self.ct_field, ct)
        setattr(instance, self.fk_field, fk)
        setattr(instance, self.cache_attr, value)


class GenericRel(ForeignObjectRel):
    """
    Used by GenericRelation to store information about the relation.
    """

    def __init__(self, field, to, related_name=None, related_query_name=None, limit_choices_to=None):
        super(GenericRel, self).__init__(
            field, to,
            related_name=related_query_name or '+',
            related_query_name=related_query_name,
            limit_choices_to=limit_choices_to,
            on_delete=DO_NOTHING,
        )


class GenericRelation(ForeignObject):
    """
    Provide a reverse to a relation created by a GenericForeignKey.
    """

    # Field flags
    auto_created = False

    many_to_many = False
    many_to_one = False
    one_to_many = True
    one_to_one = False

    rel_class = GenericRel

    def __init__(self, to, object_id_field='object_id', content_type_field='content_type',
            for_concrete_model=True, related_query_name=None, limit_choices_to=None, **kwargs):
        kwargs['rel'] = self.rel_class(
            self, to,
            related_query_name=related_query_name,
            limit_choices_to=limit_choices_to,
        )

        kwargs['blank'] = True
        kwargs['on_delete'] = models.CASCADE
        kwargs['editable'] = False
        kwargs['serialize'] = False

        # This construct is somewhat of an abuse of ForeignObject. This field
        # represents a relation from pk to object_id field. But, this relation
        # isn't direct, the join is generated reverse along foreign key. So,
        # the from_field is object_id field, to_field is pk because of the
        # reverse join.
        super(GenericRelation, self).__init__(
            to, from_fields=[object_id_field], to_fields=[], **kwargs)

        self.object_id_field_name = object_id_field
        self.content_type_field_name = content_type_field
        self.for_concrete_model = for_concrete_model

    def check(self, **kwargs):
        errors = super(GenericRelation, self).check(**kwargs)
        errors.extend(self._check_generic_foreign_key_existence())
        return errors

    def _check_generic_foreign_key_existence(self):
        target = self.remote_field.model
        if isinstance(target, ModelBase):
            fields = target._meta.virtual_fields
            if any(isinstance(field, GenericForeignKey) and
                    field.ct_field == self.content_type_field_name and
                    field.fk_field == self.object_id_field_name
                    for field in fields):
                return []
            else:
                return [
                    checks.Error(